remove std::experimental usage

C++17 moved this support out of std::experimental and into std
directly.  Fix up the code and dependencies to reflect this.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I1cd852831fc2608bd3a36f047aa6ac14741da270
diff --git a/user_channel/file.hpp b/user_channel/file.hpp
index 2c537b1..76a03c7 100644
--- a/user_channel/file.hpp
+++ b/user_channel/file.hpp
@@ -2,13 +2,13 @@
 
 #include <stdio.h>
 
-#include <experimental/filesystem>
+#include <filesystem>
 namespace phosphor
 {
 namespace user
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 /** @class File
  *  @brief Responsible for handling file pointer